Phenoxy acid herbicides are members of a family of chemicals related to the growth hormone indoleacetic acid (IAA). When sprayed on a field of crops such as wheat, rice or corn (monocots), phenoxy acid herbicides selectively induce rapid, uncontrolled growth in broad-leaf weeds (dicots) that eventually kills the unwanted vegetation and leaves the crops relatively unaffected. Phenoxy acid herbicides were independently developed in the USA and UK during World War II and were first introduced commercially in 1946. Today, 60 years later, the phenoxy acid herbicides still remain among the most widely used herbicides in the world.
There is a wide variety of phenoxy acid herbicides in use, further grouped into the phenoxyacetic, phenoxybutyric, and phenoxypropionic subtypes, the last itself containing the aryloxyphenoxypropionic subtype, which has the greatest number of commercial variants.
2,4-D_(2,4-dichlorophenoxyacetic acid) is one well-known example of a phenoxy acid herbicide. The present invention will be exemplified using this herbicide, though the other phenoxy acids can equally well be used in the same types of formulations for the same purposes. 2,4-D acid is a white, crystalline solid, minimally soluble in water, generally formulated as soluble concentrates or emulsifiable concentrates in order to facilitate its application. The soluble concentrates are usually non-volatile, water-soluble formulations of 2,4-D amine salts such as dimethylamine, isopropylamine, triethylamine, or diethanolamine salts. The emulsifiable concentrates are formulations of, for example, 2,4-D esters with high volatility, such as ethyl, propyl, isopropyl, butyl, isobutyl, or amyl esters, or 2,4-D esters with low volatility, such as butoxyethyl or 2-ethylhexyl esters.
When the term “ammonium” is used herein to refer to salts of phenoxy acids, this term applies strictly to inorganic ammonium, i.e., NH4+, unless the context demands otherwise. Phenoxy acid rates and concentrations given herein, even where the phenoxy acid is present as a salt or salts, are expressed as acid equivalent (ae)—by acid equivalent is meant that portion of a formulation that, theoretically, could be converted back to the parent acid and represents the original acid portion of the molecule—unless the context demands otherwise.
It has been generally accepted that, with the same 2,4-D acid equivalent, the 2,4-D esters are more effective than the 2,4-D amine salts, although their herbicidal effect is slower. The highly volatile esters are also more effective than esters with low volatility, but can cause undesired damage to the surrounding environment because of their volatility. The risk of unwanted damage caused by volatilisation has caused the application of highly volatile ester formulations to be regulated and restricted.
A commonly practised way to enhance the performance of pesticide products is to add an adjuvant either to the pesticide formulation or to the spray tank just before application. An adjuvant can maximise the activity of the pesticide product by a variety of functions, such as increasing spray droplet retention on difficult to wet leaf surfaces, or facilitate penetration of the pesticide into the plant cuticle. It is noted that not all adjuvants work the same way and depending on the adjuvant chosen, a certain herbicide will have a specific efficacy. Therefore for the various herbicides specific combinations with selected adjuvants have been developed and marketed.
Substances traditionally utilised as adjuvants are, for example, petroleum or natural based oils, inorganic salts, polymers, polyols, and surfactants. Surfactants have proved to be very useful and versatile adjuvants for many applications, but selecting the optimum surfactant system and the optimum concentration for a specific pesticide application is often a challenge. More specifically, as is well-known in the art, only particular surfactants work efficiently with specific herbicides. This is very clear from Ullmann's Encyclopedia of Industrial Chemistry 6th edition (Wiley Interscience; electronic edition) Chapter 7, “Chemical Control of Weeds” Part 7.2.1, which says “Intensive research with surfactants has usually shown that a particular surfactant is ideal for a specific herbicide on a particular species. This of course greatly complicates the problems involved in making recommendations for herbicides that might be used on a wide range of weed species. As a result, recommendations usually refer to a small number of common surfactants shown to be reasonably effective over a wide range of herbicides and species. Household detergents are designed for an entirely different purpose, and are usually considerably less effective as adjuvants for herbicide sprays.”
Specific amines that have proven to be useful as a pesticide adjuvant are amines with a primary, secondary or tertiary amine function which react with an acid to form a salt. By using an amine surfactant to neutralise all, or a part of, the 2,4-D acid, it is possible to create a highly concentrated, water-soluble 2,4-D formulation with a built-in adjuvant system.
U.S. Pat. No. 3,276,856 discloses compositions containing dimethyl-(C12-C18 alkyl)amine salts of phenoxy acid herbicides, e.g. 2,4-dichlorophenoxyacetic acid. These compositions have a high level of active herbicidal ingredient and improved emulsification properties, and are used to make water-in-oil emulsions.
US 2005/0215434 teaches to use herbicidal 2,4-D-amine salts, e.g. dimethylamine or diethanolamine salts, in combination with a humectant, such as ethoxylated fatty amines or amine oxides, an anti-freeze, and an anti-foaming agent in order to make liquid compositions that are non-volatile, soluble in water, and stable at low temperatures.
WO 00/64257 discloses various amine-containing surfactants—herbicide combinations, but not the alkoxylated asymmetric amines according to the invention.
In an article by L. L. Jansen in Weeds (1965), 13(2), 123-130, various amine salts of 2,4-dichlorophenoxyacetic acid are disclosed and their herbicidal activity investigated by greenhouse evaluation. Fatty amines, such as coco, soya, oleyl, and tallow alkylamine, were used as such or as ethoxylated or propoxylated derivatives. Further amine derivatives used were di(long chain alkyl) amines, such as di-coco and di-(H-tallow alkyl)amine, tertiary amines such as methyl-di-(coco alkyl)amine and dimethyl-(coco alkyl)amine, and N-alkyl-1,3-propane-diamines, such as N-oleyl-1,3-propanediamine and N—(C19 alkyl)-N,N′-diethyl-1,3-propanediamine. The salts were used in water and/or oil.
As disclosed by Jansen, alkylamine based adjuvants have been used in the past and have proven to have bioefficacy enhancing ability in regard of 2,4-D. The choice of surfactant can be important, since there are wide variations among surfactants in terms of their ability to enhance the herbicidal efficacy of phenoxy acids for particular applications. The class of surfactants known as alkylamine ethoxylates has been found particularly useful in providing enhanced efficacy to phenoxy acids. One representative alkylamine surfactant is tallowamine di-ethoxylate with 15 EO, as represented by the following chemical structure:

However, the use of these kinds of ethoxylated alkylamines as adjuvants is under scrutiny in many jurisdictions due to their unfavourable biodegradation rating, and finding a suitable adjuvant with good environmental properties in addition to a good efficacy enhancing property is difficult.
Accordingly, it is desired to develop a stable aqueous phenoxy acid salt formulation (i) having high phenoxy acid ae loading, (ii) which is stable and provides better efficacy than that of commercial phenoxy acid salt formulations, (iii) and which has an overall better biodegradability than formulations utilising present adjuvants.
These and other objectives are met by the adjuvants and herbicidal formulations of the present invention.